This invention relates generally to feedthrough capacitor terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia), neuro-stimulators, internal drug pumps, cochlear implants, ventricular assist devices, and other medical implant applications, to decouple and shield undesirable electromagnetic interference (EMI signals) signals from the device. More specifically, this invention relates to materials and methods of manufacturing monolithic ceramic feedthrough capacitors so that they can be exposed to body fluid.
It is well known in the art that EMI feedthrough capacitors can be attached to the flanges of human implantable hermetic seals for reliable EMI filter performance. These EMI filters are very important to bypass and attenuate RF signals from undesirable emitters, such as cell phones, microwave ovens and the like.
These devices are generally designed with one or more monolithic ceramic feedthrough capacitors or monolithic ceramic rectangular chip capacitors designed to be in intimate relation with the hermetic terminal. In general, monolithic ceramic capacitors are considered to be sensitive electronic components and are not manufactured of biocompatible materials. Monolithic ceramic capacitors are typically constructed of a barium titinate dielectric into which active and ground electrode plates are interspersed. It is common in the art that the ceramic capacitor dielectric be of barium titinate, zirconium titinate, or other high dielectric constant ceramic materials with various dopants added to control its dielectric constant, temperature stability and electrical properties. Barium titinate in itself is biocompatible; however, the electrodes and the termination materials are generally not biocompatible. Typical monolithic ceramic capacitors would include a palladium-silver, or nickel silver electrode system (base metal electrode). Other electrode systems are possible, including ternary, which is a high fire system consisting of an alloy of gold, platinum and palladium.
Typical capacitor termination materials are applied in two ways. The first system involves a glass frit, which is loaded with metallic particles along with a binder and vehicle system to make a paste. This paste is then applied to the capacitor and fired into place. The conductive particles make contact to the exposed electrode plates and place them in parallel. A properly formed capacitor termination is a highly conductive surface to which one can make electrical connections through soldering or other methods. Typical materials used for this glass frit are a silver or copper loaded glass frit or a palladium silver or platinum silver composition. Silver is relatively inexpensive and highly conductive and is also available in a wide variety of flakes and spherical shapes. Accordingly, it is well known in the art to build a monolithic ceramic capacitor using such termination material.
The second methodology involves plating of the termination. There are a number of plating methods currently used in the art, including a barrier plating technique which consists of plating down nickel and then various materials on top of the nickel to promote solderability. The nickel acts as a barrier layer and prevents leaching off of the capacitor. For example, if tin or copper were plated on top of the nickel, the tin or copper would readily wet with solder and the nickel would form a layer resistant to leaching or removal from the capacitor. Therefore, in nearly all of the prior art devices the monolithic ceramic capacitor is placed on the inside of the implantable medical device. In other words, this places the sensitive monolithic ceramic capacitor away from the body fluid so that it cannot come in contact with the body fluid. Another way of stating this is that a hermetic terminal is used to prevent intrusion of body fluid into the interior of the electronic device. Accordingly, all of the electronic circuits, including the substrate, circuit boards, battery, computer chips, capacitors and electromagnetic interference capacitors, are placed in a suitable location inside the titanium housing of the implantable medical device so that they are protected from body fluids.
However, modern pacemakers and implantable defibrillators tend to be very small in size and very cramped in terms of space inside the unit. Thus, placing the capacitor on the outside of the housing increases the volumetric efficiency of the overall design, such as by allowing a larger battery to be inserted in the device housing. In addition, laser welds used to seal the housing, typically comprised of titanium, will have a lesser effect on the capacitor.
Recognizing this, U.S. Pat. No. 6,055,455 discloses a monolithic ceramic capacitor placed on the outside (or the body fluid side) of the hermetic terminal of an implantable medical device. In this patent the concept of decoupling the EMI before it gets to the inside of the pacemaker or the implantable medical device is emphasized. However, it makes no difference from a filter effectiveness point of view whether the capacitor is on the inside surface or on the outside surface of the hermetic seal.
Electromagnetic interference consists of a number of modulated carrier frequencies, for example, the carrier frequency of a typical cellular phone. What is important is that the gap between the feedthrough capacitor and the hermetic seal be a wave-guide beyond cut off. In other words, that gap needs to be small enough so that the wavelength of the electromagnetic interference will not readily pass through it. As it turns out, after doing wave-guide calculations, this is relatively easy to do for a medical implant application. One reason for this is the human body's tendency to reflect and absorb EMI at frequencies of 3 GHz and above. In other words, it really makes no difference whether the EMI feedthrough capacitor is on the body fluid side or the inside of the hermetic terminal of an implantable medical device. The closely spaced feedthrough capacitor presents such a small wave-guide that it would take frequencies in excess of 20 GHz to effectively re-radiate around the filter. However, as previously mentioned, at frequencies of 3 GHz and above the human body is so effective in attenuating such signals that higher frequencies are really not of importance.
A significant mistake found in the prior art is the notion that adding some sort of an adjunct sealant over the top of a monolithic ceramic feedthrough capacitor will allow it to operate in the presence of body fluids. Body fluid is an extremely corrosive and conductive medium. There are many dissolved minerals in body fluid, including salt and potassium, which readily conduct electricity in their ionic state. Polymers and adjunct sealants and conformal coatings on electronic components have a number of weaknesses which include problems with adhesion and also bulk permeability. Simply stated, over a long period of time moisture can still penetrate through any adjunct non-hermetic sealant and eventually reach the capacitor. In addition, adjunct sealants and coatings have a different thermal coefficient of expansion as compared to the barium titinate ceramic capacitor. Thus, after exposure to temperature excursions or simply after a long period of time, the adhesion of the coating to the capacitor surface starts to break down. This could allow a thin film of moisture or body fluid to be present at the surface of the ceramic capacitor. In fact, any slight separation of any of the adjunct sealant could cause a small gap or tightly spaced separation into which moisture could easily form. One way that moisture can form in such a tiny space is through dew point condensation. That is, during temperature excursions moisture laden or vapor laden air could enter such a small separation and then deposit out as a thin film of moisture.
One of the most common and severe failures of electronic components comes from a process known as metal migration, whisker formation or dendritic growth. A dendrite can form of various migratable materials, including silver, tin, and the like. Another common way of describing this phenomenon is through tin or silver whiskers. Once these dendrites form across the surface of the capacitor, the capacitor's insulation resistance drops dramatically. This can short out the capacitor, thereby shorting out the entire implantable medical device. The effect could also be degraded insulation resistance, which could result in reduced battery life or in reduced functionality of the output waveform of the implantable medical device.
FIG. 1 is a cross-sectional view of a prior art unipolar capacitor 10, similar to that described by U.S. Pat. No. 4,424,551, the contents of which are incorporated herein. At first glance it would appear that the capacitor 12, shown inside the ferrule 14, is well protected against body fluid by the sealant 16, such as an epoxy seal. However, in actual practice there is a mismatch of thermal coefficient of expansion between the polymers and the barium titinate of the ceramic capacitors. There are also adhesion problems and difficulties with bulk permeability. Accordingly, across both the top and bottom surfaces of the capacitor 12 one can usually see, at high magnification, a small separation 17 is often present between the sealing material and the capacitor surface itself. This would be a separation on the top surface of the capacitor 12 and sealing material 16 due to a separation in the bond between non-conductive sealing material 16 and the capacitor 12. After a prolonged period of time, moisture can penetrate into either one of these spaces. Accordingly, a metal migration or dendrite 18 can form either on the top or bottom of the capacitor 12. As mentioned above, the formation of this dendrite could lead to either immediate or latent catastrophic failure of the implanted medical device.
With reference to FIGS. 3–5, a prior art unipolar feedthrough capacitor 20 mountable to a hermetic terminal of an implantable medical device, such as a cardiac pacemaker, an implantable cardioverter defibrillator (ICD) a cohlear implant, or the like is shown. Such prior art capacitors 20 are typically constructed using a silver-bearing or palladium silver bearing-glass frit for the outside diameter termination surface 22 as well as the inner diameter surface 24. Connecting material 26 connects the capacitor's lead wire 28 to the inside diameter surface 24 of the feedthrough capacitor 20. The material 26 is typically of a silver-filled conductive polyimide, or a lead or tin bearing solder or the like. If the capacitor 20 were exposed and placed on the body fluid side of the medical device, a thin film of moisture 30 would be present across the surface of the capacitor. This moisture could be present from direct immersion in body fluid or from the penetration of any adjunct sealants by body fluids. In the presence of moisture 30, dendrites or metal migration 32 would form or grow between the areas of opposite polarity 22 and 24. This dendritic growth or migration can also occur from the capacitor's outside diameter metallization material and the material used to make the electrical mechanical connection between the capacitor lead wire 28, and the capacitor's inside diameter 24. Even if the capacitor's outside diameter termination material 22 was of biocompatible material, (which is not typical in the prior art), the connection material 26 which forms the electromechanical connection from the capacitor outside diameter 22 to a ferrule 34, could still be problematic. That is due to the fact that the connecting material 26 is typically a silver-filled conductive thermosetting polymer, such as a conductive polyimide or the like.
Thus, in the presence of moisture and a voltage bias, the silver is free to migrate and form dendrites 32 as shown in FIGS. 3 and 5. Of course those skilled in the art will realize that the formation of these dendrites 32 is highly undesirable because they are conductive and tend to lower the insulation resistance or short out the capacitor 20. This is particularly problematic in a low voltage pacemaker application. In cardiac pacemaker applications, the formation of the silver, tin or other dendrites 32 would preclude the proper operation of the implanted medical device. Another undesirable effect of the formation of these dendrites 32 is that they would tend to conduct current and thereby dissipate power unnecessarily, leading to premature battery failure of the implanted medical device. Premature battery failure is highly undesirable and leads to unwanted surgery and increased expense, usually the replacement of the entire implantable medical device.
With reference now to FIGS. 6 and 7, a surface mounted quadpolar capacitor 36 is illustrated, such as that described in U.S. Pat. No. 5,333,095, the contents of which are incorporated herein. As can be seen from the illustration, dendrites 38 or 38′ can form between any points of opposite polarity as long as there is migratable material as well as a migratable medium. As previously mentioned, migratable mediums include thin films of moisture, solvents or the like. Accordingly, another problem can arise during cleaning or washing of the capacitor 36. Any entrapped cleaning solvents, such as alcohol, water or degreasers along with a bias voltage can allow for the migration of the metallic migratable materials. It will be appreciated by those skilled in the art that not only can the dendrites 38 form between lead wires of opposite polarity 40, but also at 38′ between two lead wires of the same polarity and an adjacent ground at the capacitor outside diameter metallization 42. Both conditions are highly undesirable in that the dendrite 38 formation could short out or reduce the insulation resistance between the two lead wires thereby degrading any biological signal sensing that they may perform. The term “short out” does not necessarily imply that the dendrite 38 will form a zero ohm connection because the resistance of the dendrite, metal migration or whisker depends upon a number of factors including the thickness density and length of the dendrite 38 or 38′ that is formed. Dendrites do not form a continuous sheet, but rather are discontinuous. Time lapse photography has shown that dendrites form side branches similar to a tree with many leaves. Accordingly, what results is a matrix of silver conductive particles that have many strange geometric shapes. Accordingly, the resistivity of such a structure is highly variable, ranging from several thousand ohms down to a very few ohms.
With reference to FIG. 8, an in-line quadpolar capacitor 44 is illustrated wherein the outside or ground termination 46 is in two localized areas. Such localization minimizes the opportunity for dendrites to form. However, when the electrical connection is made between the termination material 46 and the conductive ferrule material 48 using a connective material 50 which is comprised of migratable material, typically a silver-filled solder or conductive thermal-setting polymer such as a conductive polyimide or the like, the formation of dendrites 52 or 52′ is possible in the presence of moisture. A dendrite 52 could form between the capacitor conductive metallization 46 and lead wire 54 or a dendrite 52′ could form between lead wires 54, as illustrated.
With reference to all of the illustrated prior art, when the capacitor is installed in the housing of an implantable medical device and the capacitor is oriented toward the inside, such dendrites typically do not form. This is because the inside of the implantable device is hermetically sealed. This prevents intrusion of body fluids or other moisture. In addition, the active implantable medical device is typically thoroughly cleaned and then baked dry prior to assembly. The device is then laser welded shut. Prior to final sealing, the interior of the implantable medical device is evacuated at high vacuum and then back-filled with dry nitrogen. In other words, the ceramic capacitors of the prior art are never really exposed to moisture throughout their design life. Accordingly, the dendrites 52 in FIG. 8 do not have a chance to form when the capacitor is oriented to the inside of a properly constructed active implantable medical device.
FIG. 10 illustrates a prior art internally grounded bipolar feedthrough filter capacitor 56, such as that disclosed in U.S. Pat. No. 5,905,627 the contents of which are incorporated herein by reference. Even though the capacitor 56 has no outside diameter or outside perimeter metallization, a dendrite 58′ can still form if a moisture film and voltage bias form between the lead wires 60 and 66 or a dendrite 58 can form between a lead wire 60′ and a conductive ferrule 62. In this case, the conductive ferrule 62 has been greatly simplified and shown as a rectangular plate. In the art, these ferrules 62 take on a variety of sizes and shapes, including H-flanges to capture the mating halves of an implantable medical device housing. As shown, the dendrite 58 has formed all the way from the conductive material to the ferrule 62 used to make the connection between the capacitor lead wire 60 and the capacitor inside diameter 64, which would typically be a conductive polyimide solder or the like. In an internally grounded feedthrough capacitor 56, there is always a grounded lead wire 66 which is connected to the capacitor's internal electrode plate set 68, illustrated in FIG. 12. It is also possible, or even likely, to form a dendrite 58′ between this lead wire and any adjacent lead of opposite polarity. Such a dendrite 58′ would short out the lead wire 60 to the grounded lead wire 66. This is why coating such leads, which may be formed of noble metal material, with migratable metals or materials such as tin-lead combinations, is problematic. Thus, it will be readily apparent by those skilled in the art that dendrites can form and migrate over any migratable conductive material, such as silver-filled conductive thermal-setting connective material which is often used to connect lead wires 60 and 66 to the inside diameter metallization 64 of the feedthrough capacitor or conductively connect the outside of the capacitor to the ferrule 62.
It should be noted that for a dendrite to form, the migratable material need not be present on both sides. In other words, a migratable material is not necessarily both the cathode and the anode. There are no materials in titanium that would migrate, however, silver particles from conductive silver bearing glass frit fired onto the capacitor is capable of migrating in the presence of a voltage bias and a moisture film. It is also possible that a dendrite material form directly between the inside diameter metallizations 64 from the ground feedthrough hole and one or more of the active insulated feedthrough capacitor wires.
Detecting the presence of these dendrites can sometimes be very confusing for the test technician. This is because the dendrites most readily form in a high-impendence, low voltage circuit where a moisture film is present along with migratable materials. The dendrite, metal migration or metal whisker is typically very lacy, thin and of low cross-sectional area. Accordingly, this material can act like a fuse and open up if a high voltage or a low impedance voltage or current source is applied. Accordingly, when dendrites are present, they are sometimes inadvertently blown open by routine electrical testing either by the manufacturer or by the customer's receiving inspection department. A concern is that after years of field use, if the dendrite were to reform, this could slowly degrade the battery life of the medical device through decreased insulation resistance or degrade the device's ability to sense very low level biological signals. These are yet again reasons why it has been common in the prior art to always place the ceramic feedthrough capacitor toward the inside where it is protected from body fluids.
FIG. 13 shows a prior art integrated chip capacitor 70, such as that described in U.S. Pat. Nos. 5,959,829 and 5,973,906, the contents of which are incorporated herein. These chip capacitors 70 come in a variety of sizes and shapes and are used to decouple electromagnetic interference from the lead wires 72 of an implantable medical device to the metallic ferrule 74. As illustrated, capacitor 70 has integrated four rectangular chip style capacitors into a single monolithic package. Each of these chip capacitors makes a connection to the lead wire 72 and decouples EMI to the metallic ferrule 74. Since prior art chip capacitors are constructed of the same materials as are typical in the entire capacitor industry, it is likely that a dendrite 76 will form if moisture or solvents are present. Such dendrites 76 can form between the migratable connective materials used to connect the capacitor metallization 78 to the lead wire 72 and the ferrule 74, or between the lead wires 72 (not shown).
It is a common misconception that it takes many months or years for metal migration or dendrites to form. Actually, the dendrite itself has been observed to form very quickly so long as a migratable material, a moisture or solvent film, and a suitable bias voltage is present. Once these three factors come together, it can be only a matter of seconds or minutes for the dendrite itself to actually form. As previously mentioned, dendrites can also form from lead wires to the conductive materials used to connect the capacitor's ground termination to the conductive ferrule. This is the case even if the ferrule is of a non-migratable material such as titanium or a noble metal, such as gold or the like, provided that the connective material is of a migratable material such as silver, tin, or other known migratable metals. As can been seen, there are many ways for such dendrites to form. Notwithstanding U.S. Pat. No. 6,055,455, the inventors are not aware of a single instance in an implantable medical device where the capacitor has been placed on the outside and exposed to body fluid. Instead, it has been standard practice in the medical implant industry that all electronic components be protected inside the hermetically sealed enclosure, which is typically vacuum evacuated and back filled with an inert gas such as nitrogen or the like to ensure a very dry atmosphere, and prohibit contact with body fluids. Of course, in such a dry atmosphere, one of the three essential ingredients for metal migration or dendrite formation is removed and such dendrites do not form.
Metal migration, whiskers and dendrite formation does not only occur of the surfaces of ceramic feedthrough and chip capacitors. Said dendrites can also form inside the capacitor along microfractures, cracks, or knit line defects (slight separations in the capacitor electrode lamination boundary). Internal metal migration within a ceramic capacitor can have the same catastrophic effects as surface migration. That is, the insulation resistance of the capacitor can be severely reduced including the shorting out of the capacitor completely.
The ceramic feedthrough capacitor which acts as an EMI filter is poised directly at the point of ingress and egress of the lead wires between the implantable medical device and body tissue. For example, in a cardiac pacemaker, the feedthrough capacitor is placed at the point where lead wires from the heart enter into the pacemaker itself. Accordingly, any short circuiting or lowering of insulation resistance of the ceramic feedthrough capacitor precludes or shorts out the proper operation of the pacemaker itself. This can be very dangerous or even life threatening to a pacemaker-dependent patient whose heart depends on each pulse from a pacemaker so that it itself will beat. There are numerous instances in the literature wherein cardiac pacemakers, implantable defibrillators and neurostimulators have been shown to adversely react in the presence of an emitter such as a cell phone or retail store security gate (electronic article surveillance system). Pacemaker potential responses to EMI include sensing (pacemaker inhibition), noise reversion to asynchronous spacing, tracking for dual chamber devices, in rate adaptive devices the rate changes within programmed rate limits, activation of the lead switch, ICD undersensing, asynchronous pacing, or microprocessor reset. In an implantable cardioverter defibrillator (ICD), potential responses to EMI can include all of the responses for a pacemaker in that ICDs often include a pacemaker function. In addition, ICDs may also respond to EMI by over-sensing that manifests itself as either inhibition or an inappropriate delivery of therapy. An inappropriate delivery of therapy means that a fully alert and cognizant patient would receive a high voltage shock. Delivery of such a high voltage can injure the patient by literally throwing him off his feet (such a case has been documented with the male patient breaking his arm). In addition, ICDs can respond to EMI by tracking, undersending an arrhythmia, or electrical current directly induced in the lead system that can trigger a dangerous cardiac arrhythmia. Accordingly, proper operation of the EMI filter is critical to protect the implantable medical device from not exhibiting any of the possible aforementioned malfunctions. Formation of dendrites can seriously degrade the proper operation of the pacemaker and/or make the filter ineffective at performing its proper function.
For example, with reference to FIGS. 15–17, a cross-sectional view of a prior art unipolar feedthrough capacitor assembly 79 is shown similar to that described in U.S. Pat. Nos. 4,424,551; 4,152,540; 4,352,951 and others. Monolithic ceramic capacitors have a relatively low thermal coefficient of expansion compared to metals. Ceramic capacitors are very strong in compression, but very weak in tension. This is typical of most brittle materials. Accordingly, it is very easy to introduce cracks within the ceramic capacitor structure if the capacitor is subjected to excessive stresses. The ceramic capacitor assembly shown in FIG. 15 has the ceramic capacitor embedded within a metallic ferrule 80. For a human implant application, this metallic ferrule 80 would typically be made of titanium and could have a variety of shapes and flanges. The connection from the inside diameter of the ferrule 80 to the outside diameter metallization 82 of the feedthrough capacitor is shown as material 84. Material 84 is typically a thermal-setting conductive adhesive, such as a silver-filled conductive polyimide, epoxy or the like. The entire assembly shown in FIG. 15 is designed to be installed into a pacemaker, ICD or the like by laser welding directly into the titanium can of the implantable device. Accordingly, the ferrule 80 is rapidly heated and tends to expand. The relatively cooler ceramic capacitor 79 does not expand nearly at the same rate. Accordingly, a variety of cracks can be introduced into the ceramic capacitor. These cracks can be axial, radial or cover sheet type features.
For purposes of example, as shown in FIG. 17, a crack 86 has propagated across the corner of the ceramic capacitor 87. Additionally, the crack 86 has contacted plates 88 and 90 of opposite polarity. In other words, the crack 86 has propagated through the main body of the ceramic dielectric 92 between a ground electrode plate 88 and the lower active electrode plate 90. This in and of itself does not present an immediate electrical defect. The reason for this is that as long as the crack 86 itself does not contain metallic particles, the two electrodes 88 and 90 are not shorted out. However, it is quite possible for these cracks to propagate to the outside diameter or top surface of the capacitor 87. Long-term exposure to body fluid in combination with the bulk permeability of the surrounding polymers can lead to the presence of a moisture thin film that lines the inside of this crack 86. FIG. 17A shows a silver dendrite 86′ that has formed by metal migration through the cracks. The reason for the formation of the dendrite has to do with the intrinsic materials that are typically used in the prior art electrodes and capacitor terminations themselves. Ceramic capacitors are typically made with nickel, silver or palladium silver electrodes. These are low cost electrode systems that are found in many ceramic capacitors today. They are formed within the solid monolithic ceramic by firing or sintering at a relatively low temperature (around 1100° C.). As previously mentioned, an internal dendrite is a highly undesirable situation to occur because this shorts out the ceramic capacitor. Such shorting or reduced insulation resistance of the ceramic capacitor not only degrades its effectiveness as an EMI filter, it also can cause the catastrophic failure of the entire implantable medical device. As mentioned, this can be life threatening, for example, in the case of a pacemaker-dependant patient. The dendrite can be low enough in resistance to short out the pacemaker output pulse. In this case, the patient's heart would simply stop beating, which would quickly lead to death.
Accordingly, there is a need for a feedthrough filter capacitor which can be disposed on the body fluid side of an implantable device to provide additional space for an enlarged battery, a smaller implantable device, etc., while being immune to dendritic growth. The present invention fulfills these needs and provides other related advantages.